As restrictions on the emission of acid rain precursors such as sulfur dioxide have grown in recent years, a corresponding need has arisen for technologies capable of removing such compounds from gaseous streams in which they are present. Examples of such gaseous streams include flue gases from incinerators and fossil-fueled power plants. It is well known to use activated carbons and cokes for the removal of sulfur dioxide from such streams, especially above 100 C. Below 100 C. and in the presence of oxygen and water, carbonaceous chars can act as catalysts for the oxidation of sulfur dioxide to sulfuric acid. However, the rate of reaction is usually so low that the loadings and concentrations of sulfuric acid which accumulate on the char are rarely of commercial interest.
The use of carbonaceous char catalysts treated with nitrogen-containing compounds during their manufacture or prepared from nitrogen-rich starting materials have shown particular promise in this application. For example, improvements in the rate of oxidation of sulfur (IV) to sulfur (VI) have been achieved in the liquid phase using high-temperature, nitrogen-poor carbonaceous chars which are post-calcined in the presence of ammonia or other nitrogen-containing substances. Improved direct gas phase oxidation of sulfur dioxide has been achieved for high temperature nitrogen-poor chars post-calcined in presence of melamine or its derivatives. Other carbonaceous chars have been used for the enhanced liquid-phase and gas-phase oxidation of sulfur (IV). These carbonaceous chars were derived from nitrogen-rich starting materials such as polyacrylonitrile. Also, a char suitable for the catalytic oxidation of sulfur (IV) has been produced wherein a high-temperature nitrogen-poor activated carbon or coke was oxidized by sulfur (VI) and exposed to a nitrogen-containing ammonia salt at temperatures above 350 C.
All of the prior art methods for improving the inherent catalytic capabilities of carbonaceous materials for sulfur (IV) oxidation have certain disadvantages which limit their overall utility. For example, almost all use high-temperature chars as starting materials. High-temperature carbonaceous chars are those produced by thermal treatment at temperatures greater than 700 C. Low-temperature chars have not experienced temperatures greater than 700 C. Since high-temperature carbonaceous chars are fairy inert chemically, the use of aggressive chemical post-treatments is usually required to effect significant changes in their catalytic capabilities. One exception is the use of an expensive synthetic starting material such as polyacrylonitrile. However, this is also disadvantageous. Additionally, the use of high-temperature finished chars as starting materials is inevitably more expensive than the direct use of the raw materials from which they are made. In some cases large quantities of toxic process byproducts such as sulfur dioxide and cyanide are unavoidably produced, while others require the use of highly hazardous treatment agents such as sulfuric acid.
Accordingly, it is the object of the present invention to provide an improved process for the removal of sulfur oxides from gas or liquid media containing oxygen and water at temperatures below 100 C. using a carbonaceous char catalyst prepared directly from a nitrogen-poor, naturally-occurring starting material such as a bituminous coal or a bituminous coal-like material. It is further the object of the present invention to limit the use of agents responsible for imparting catalytic activity to the carbonaceous char by performing the essential treatments during the low-temperature (less than 700 C.) transition of the starting material to the final product. These treatments include low-temperature (less than 700 C.) oxidation of the starting material to produce a low-temperature oxidized char, preferably by inexpensive, abundant, relatively non-toxic oxidants such as air, and exposure of the oxidized low-temperature char to nitrogen-containing compounds during, not after, calcination and condensation of the carbon structure. The preferred nitrogen-containing compounds are inexpensive, abundant, and relatively non-toxic agents such as urea or melamine but other nitrogen-containing compounds may also be used. By this method, carbonaceous chars with high catalytic activity per se for the oxidation of sulfur (IV) in the presence of oxygen and water can be produced with minimal departure from conventional processes for the production of high-temperature carbonaceous chars such as activated carbons and cokes.